A PV device converts the energy of sunlight directly into electricity by the photovoltaic effect. The PV device can be, for example, a PV cell, such as a crystalline silicon cell or a thin-film cell. PV modules can include a plurality of PV cells or devices. PV cells can include multiple layers created on a substrate (or superstrate). For example, a PV device can include a transparent conductive oxide (TCO) layer, a buffer layer, and semiconductor layers formed in a stack on a substrate. The semiconductor layers can include a semiconductor window layer formed on the buffer layer and a semiconductor absorber layer formed on the semiconductor window layer. Additionally, each layer can cover all or a portion of the device and/or all or a portion of the layer or substrate underlying the layer. For example, a “layer” can include any amount of any material that contacts all or a portion of a surface.
FIG. 1 is a cross-sectional view of a conventional PV device 10 typically formed sequentially in a stack on a substrate 110, e.g., soda-lime glass. Because substrate 110 is not conductive, PV device 10 can include a front contact 120, which can include a multi-layered TCO stack with several functional layers including a barrier layer to protect the semiconductor layers from potential contaminants, a TCO layer, and a buffer layer to mitigate potential irregularities during the formation of the semiconductor layers, for example. The semiconductor layers can include an n-type semiconductor window layer 130, such as a cadmium sulfide (CdS) layer, formed on the front contact 120 and a p-type semiconductor absorber layer 140, such as a cadmium telluride (CdTe) layer, formed on the semiconductor window layer 130. The window layer 130 can allow the penetration of solar energy to the absorber layer 140, where the optical energy is converted into electrical energy. Specifically, the n-type window layer 130 may contact the p-type absorber layer 140 to form a p-n junction. As a result of diffusion across the junction, negative acceptor ions are formed on the p-type side and positive donor ions are formed on the n-type side. The presence of the ions creates a built-in electric field across the junction. When a photon is absorbed within the p-n junction, an electron-hole pair is formed. Movement of the electron-hole pairs are influenced by the built-in electric field, which produces current flow between the front contact 120 and a back contact 150. Back contact 150 is formed over absorber layer 140. Back contact 150 may be a low-resistance ohmic contact. Front and back contacts 120, 150 may serve as electrodes for transporting photocurrent away from PV device 10. Back support 160, which may be glass, is formed over back contact 150 to protect PV device 10 from external hazards.
During fabrication, absorber layer 140 can be formed (or deposited) on window layer 130 by a vapor transport deposition (VTD) distributor system, for example, and then heat treated. After deposition of absorber layer 140, a back surface 149 of absorber layer 140 may have a surface roughness quantified by the arithmetic mean value (Ra, a measurement of the average roughness of a surface calculated based on the height variations of the surface) that is smooth, e.g., less than about 10 nm, which could affect adhesion of the back contact 150 to the absorber layer 140. Additionally, a smooth back surface 149 of absorber layer 140 could potentially inhibit CdTe grain growth and doping uniformity during post-deposition heat treatment. CdTe grain growth can produce a larger CdTe grain size in the CdTe layer which may increase carrier (e.g., electron, hole) mobility within the p-n junction and thus may boost the electrical output of PV device 10.
A semiconductor absorber layer of a PV device having a greater back surface roughness is desirable.